"Over the past few weeks we've been working with Microsoft to lay plans for the future of State of Decay, including future Title Updates, DLC, and possible sequels. As part of that process we've also been thoroughly evaluating the possibility of adding co-op multiplayer to State of Decay. While there are great things in store for State of Decay, unfortunately multiplayer is not one of them."

I think they aren't going to do this because the sequel game is going to be the full MMO they talked about. You might as well just play that if you want co-op in their mind. The MMO was their endgame from the start also.